Electrostatic imaging paper

ABSTRACT

An electrically conductive base paper has on one side thereof a continuous coating of dielectric material consisting of a polymeric material deposited from an aqueous dispersion having a minimum film-forming temperature not greater than 30*C to provide an intense image with no background discolouration due to pick-up of excess toner material during processing. If desired two such coatings may be used of which each has a minimum film-forming temperature not greater than 30*C or the second of which has a minimum film-forming temperature of between 60*C and 120*C. The coating may contain an inorganic pigment material in an amount of up to about 20 percent by weight of dry polymeric material used.

United States Patent [191 Hill et al.

[ Nov. 12, 1974 ELECTROSTATIC IMAGING PAPER Inventors: Andrew William 11111, Great Missenden; Paul Donald Sims, Hazlemere, both of England Assignee: Wiggins Teape Research &

Development Limited, London,

England Filed: Dec. 12, 1972. App1. l\lo.: 314,435

Foreign Application Priority Data Dec. 20, 1971 Great Britain 59082/71 U.S. Cl... 117/218, 117/201, 252/500, 96/15, 96/18 Int. Cl B44d l/l6, (303g 5/04 Field of Search 117/201, 218; 96/15, 1. 8; 252/500 References Cited 7 UNITED STATES PATENTS Levy 117 201 Yamaguchi 96/1 8 3/1973 Honjo 117/218 3,720,514 3,652,271 3/1972 Bomarth... 96/].8 3,586,532 6/1971 Freed .1 117/201 Primary Examiner-Mayer Weinblatt Assistant Examiner-M. F. Esposito Attorney, Agent, or Firm--Burns, Doanc, Swecker & Mathis [57] ABSTRACT forming temperature of between 60C and 120C. The

coating may contain an inorganic pigment material in an amount of up to about 20 percent by weight of dry polymeric material used.

23 Claims, No Drawings ELECTROSTATIC IMAGING PAPER This invention relates to an electrostatic imaging paper comprising an electrically conductive base paper having on one sidethereof a continuous coating of dielectric material, and to methods of manufacturing such paper.

Such paper is used in electrostatic imaging systems in which an electrical charge pattern is applied to the dielectric material coating, for example by means of a stylus, the charge pattern then being rendered visible by the application of a so-called toner material, either as a dry powder or as a powder dispersion in a nonaqueous liquid, with subsequent fixing to render the visible pattern permanent.

Various methods of manufacturing such paper are well known, and with regard to the dielectric material coating, this is usually provided either by extruding a polymeric material layer onto an electrically conductive base paper, or by coating onto an electrically conductive base paper 'a non-aqueous solvent solution of a polymeric material.

It has also been suggested that the dielectric material coating be provided by coating onto. an electrically conductive base paper either an aqueous solution, or an aqueous dispersion (e.g. a latex), of a polymeric material. The use of an aqueous coating method is preferred due to the relative cheapness and simplicity thereof, but known aqueous coating methods have the disadvantage that the paper produced thereby has a poorer performance in use than paper produced by the more common'extrusion and solvent coating methods.

The properties required of a dielectric material coating are that it is preferably capable of being applied to a base paper using conventional coating apparatus to form a smooth continuous film; the coating should have dielectric properties which are independent of the humidity of the surroundings, that is the coating should be resistant to water vapour; the coating should be nontacky and non-yellowing at all temperatures likely to be encountered in use; and in use the coating should provide an intense, well-defined image with no background discolourations due to pick-up of excess toner material during processing.

According to one aspect of this invention, in an electrostatic imaging paper comprising an electrically conductive base paper having on one side thereof a contin: uous coating of dielectric material, the dielectric material coating is of a polymeric material deposited from an aqueous dispersion having a minimum film-forming temperature not greater than 30C.

Preferably the dielectric material coating contains an inorganic pigment material in an amount up to about 20 percent by weight of dry polymeric material used.

The inclusion of an inorganic pigment material, such as titanium dioxide, barium sulphate, silica, clay, or zinc oxide, reduces the tackiness of the dielectric material coating.

According to another aspect of the present invention a method of manufacturing an electrostatic imaging paper comprising an electrically conductive base paper having on one side thereof a continuous coating of dielectric material, includes the steps of applying to one side of an electrically conductive base paper an aqueous dispersion of a polymeric material, said dispersion having a minimum film-forming temperature not greater than 30C, and drying the coated base paper to form thereon a continuous coating of the polymeric material.

An advantage obtained by the use of a polymeric material deposited from an aqueous dispersion having a minimum film-forming temperature of 30C or lower is that migration of the conductivity agent contained in the base paper into the dielectric material coating during application of the dielectric material coating is kept to a minimum and thus the electrical resistivity of the dielectric material coating is not affected too deleteriously.

As mentioned above the important properties which the dielectric material coating should have are that it should be smooth and continuous, the smoothness being desirable to prevent the pick-up of excess toner material during processing, and the continuity in the coating e.g. absence of pinholes, being desirable to prevent discharge of an applied electrical charge pattern through these discontinuities to the conductive base paper and further to prevent liquid toner material penetrating through these discontinuities and discolouring the base paper.

Thus, the method of this invention can include the further step of applying over the dried first aqueous dis persion coating a second aqueous dispersion of a polymeric material.

If the paper is to be .used in a printer employing a dry toner material, it is preferable that the paper has a second coating which is non-tacky so that there is less background pick-up of toner.

For this use, therefore, the second aqueous dispersion preferably has a minimum film-forming temperature higher than 30C, for example between 60 and C.

In the method of this invention the or each aqueous dispersion used can include pigment material as mentioned above.

Various commercially available polymeric materials are suitable for use in carrying out this invention, and the selection of a material for any particular application will normally be governed by the physical properties required and the cost rather than by electrical properties which are generally similar. Typical polymeric materials which can be used are the polyolefms and substituted polyolefins, and condensation polymers such as polyesters and phenolic resins. However, if the imaging paper is to be usable in extreme humidity conditions then the polymeric material coating should ideally contain no ionic or ionisable groups and should have a low affinity for water.

Similarly various commercially available pigment materials are suitable for use in carrying out this invention.

To prevent settling of the pigment and aid dispersion thereof in the aqueous dispersion of polymeric material, a pigment dispersant may be required. However, most known pigment dispersants are ionic and thus would have an adverse effect on the humidity properties of the formed dielectric material coating. It has been found that ammonium based salts, e.g. the ammonium salt of a polycarboxylic acid, are suitable for use as pigment dispersants since on heating such a salt, ammonia is driven off leaving a non-ionic form of the dispersant in the formed dielectric material coating.

This invention will now be described by way of example with reference to specific workings thereof.

EXAMPLE 1 A dielectric material aqueous dispersion coating mix was prepared to the following recipe:

A styrene acrylic polymeric material dispersion having a minimum filmforming temperature of 27C; supplied under the name Vinacryl 7170 by Vinyl Products Ltd.

A high surface area silica pigment material; supplied under the name Gasil 16 by J. Grosfield & Sons Ltd.

An anti-foaming agent; supplied under the name Foamester B by Nopco Hess Ltd.

EXAMPLE II A first aqueous dispersion coating mix was prepared to the following recipe:

Styrene acrylic polymeric material dispersion having a minimum film-forming temperature of 27C (Vinacryl 7170) as in Example 1 40 kg Silica pigment material (Gasil l6) as in Example I kg An anti-forming agent: supplied under the name Nopco 8034 by Nopco Hess Ltd. 60 kg A pigment dispersant in the form of an ammonium salt of a polycarboxylic acid; supplied under the name Dispex A40 by Allied Colloids Manufacturing Co. Ltd. 67 ml Water: 93 kg A second aqueous dispersion coating mix was prepared to the following recipe:

Styrene acrylic polymeric material (Vinacryl 7170) as in Example I 13.6 kg

A polystyrene polymeric material dispersion having a minimum film-forming temperature of between 100C and 120C; supplied under the name Vinamul 7700 by Vinyl Products Ltd. 27.2 kg

Silica pigment material (Gasil 16) as in Example I 3.6 kg

Anti-foaming Agent (Nopco 8034) as in first mix 60 ml Pigment dispersant (Dispex A40) as in first mix 48 ml Water 15.6 kg

The first mix was coated on to an electrically conductive base paper as in Example 1 to give a coating weight of about 7 g/m, and the applied coating dried at a temperature of about 100C.

The second mix was then coated over the first coating as in Example I to give a coating weight of about 5 g/m, and the applied coating dried at a temperature of about 120C.

EXAMPLE III In this Example, two coatings were applied as in Example II, but the polymer dispersion of both the first and the second coating mixes had minimum filmforming temperature below 30C. The first aqueous dispersion coating mix was prepared to the following recipe:

An acrylic polymeric material dispersion having a minimum filmforming temperature of approximately 20C; applied under the name Acronal 290D by Badische Anilin and Soda Fabrik A.G. 50 kg Barium Sulphate 4.39 kg Anti-Foaming Agent (Nopco 8034) as in Example I ml Pigment dispersant (Dispex A40) as in Example 11 55 ml Water 19.2 kg

The second aqueous dispersion coating mix was prepared to the following recipe:

Styrene acrylic polymeric material dispersion having a minimum filmforming temperature of 27C The first mix was coated on to an electrically conductive base paper as in Examples 1 and 11 to give a coating weight of about 3 g/m and the applied coating dried at a temperature of about C.

The second mix was then coated over the first coating as in Examples I and II to give a coating weight of about 3 g/m and the applied coating dried at a temperature of C.

The electrostatic imaging paper produced in the above Examples was tested by comparison of performance with commercially available papers produced using solvent coating techniques, and was found to be satisfactory.

We claim:

1. An electrostatic imaging paper comprising an electrically conductive base paper having on one side thereof a continuous coating of dielectric material, said dielectric material coating being of a polymeric material deposited from an aqueous dispersion which has a minimum film-forming temperature not greater than 30C and wherein said polymeric material is selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers.

2. The electrostatic imaging paper of claim 1 wherein said dielectric material coating also contains an inorganic pigment material in an amount up to about 20 percent by weight of said dry polymeric material used, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.

3. The electrostatic imaging paper of claim 2 wherein said dielectric material coating also contains a pigment dispersant, said pigment dispersant being an ammonium salt.

4. The electrostatic imaging paper of claim 1 having applied over said continuous coating of dielectric material a second coating deposited from a second aqueous dispersion of a polymeric material, in which said polymeric material is selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, containing no ionic or ionizable groups and having a low affinity for water.

5. The electrostatic imaging paper of claim 4 wherein said second aqueous dispersion has a minimum filmforming temperature higher than 30C.

6. The electrostatic imaging paper of claim 4 wherein said second aqueous dispersion has a minimum filmforming temperature between 60 and 120C.

7. The electrostatic imaging paper of claim 4 wherein said second coating contains an inorganic pigment material in an amount up to about 20 percent by weight of dry polymeric material, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.

8. The electrostatic imaging paper of claim 7 wherein said second coating also contains a pigment dispersant, said pigment dispersant being an ammonium salt.

9. The electrostatic imaging paper of claim 1 wherein said polymeric material contains no ionic or ionizable groups and has a low affinity for water.

10. The electrostatic imaging paper of claim 1 wherein said polymeric material is selected from the group consisting of acrylic and styrene-acrylic polymeric material.

11. A method of manufacturing an electrostatic imaging paper comprising an electrically conductive base paper having on one side thereof a continuous coating of dielectric material, said method including the steps of (a) applying to one side of an electrically conductive base paper an aqueous dispersion which has a minimum film-forming temperature not greater than 30C, wherein said polymeric material is selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, and (b) drying the coated base paper to form thereon a continuous coating of the polymeric material.

12. The method of claim 11 wherein said aqueous dispersion of polymeric material contains an inorganic pigment in an amount up to about 20 percent by weight of dry polymeric material used, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.

13. The method of claim 12 wherein a pigment dispersant is used to aid dispersion of the pigment material in said aqueous dispersion of polymeric material, said pigment dispersant being an ammonium salt.

14. The method of claim 11 wherein said base paper is (a) further coated with a second aqueous dispersion of a polymeric material, said polymeric material being selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, and (b) dried to form thereon a second continuous coating of dielectric material.

15. The method of claim 14 wherein said second aqueous dispersion has a mimimum film-forming temperature higher than 30C.

16. The method of claim 14 wherein said second aqueous dispersion has a minimum film-forming temperature of between 60 and 120C.

17. The method of claim 14 wherein said second aqueous dispersion contains an inorganic pigment material in an amount up to about 20 percent by weight of dry polymeric material, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.

18. The method of claim 17 wherein a pigment dispersant is used to aid dispersion of the pigment material in said second aqueous dispersion of said polymeric material, said pigment dispersant being an ammonium salt.

19. The method of claim 11 wherein said polymeric material contains no ionic or ionizable groups and has a low affinity for water.

20. The method of claim 11 wherein the coated base paper is dried at a temperature of substantially C.

21. The method of claim 14 wherein said polymeric material in said second aqueous dispersion contains no ionic or ionizable groups and has a low affinity for water.

22. The method of claim 14 wherein said coated paper is dried at a temperature of substantially C.

23. The method of claim 11 wherein said polymeric material is selected from the group consisting of acrylic and styrene-acrylic polymeric material. 

1. AN ELECTROSTATIC IMAGING PAPER COMPRISING AN ELECTRICALLY CONDUCTIVE BASE PAPER HAVING ON ONE SIDE THEREOF A CONTINUOUS COATING OF DIELECTRIC MATERIAL, SAID ELECTRIC MATERIAL COATING BEING OF A POLYMERIC MATERIAL DEPOSITED FROM AN AQUEOUS DISPERSION WHICH HAS A MINIMUM FILM-FORMING TEMPERATURE NOT GREATER THAN 30*C AND WHEREIN SAID POLYMERIC MATERIAL IS SELECTED FROM THE GROUP CONSISTING OF POLYOLEFINS, SUBSTI TUTED POLYOLEFINS AND CONDENSATION POLYMERS.
 2. The electrostatic imaging paper of claim 1 wherein said dielectric material coating also contains an inorganic pigment material in an amount up to about 20 percent by weight of said dry polymeric material used, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.
 3. The electrostatic imaging paper of claim 2 wherein said dielectric material coating also contains a pigment dispersant, said pigment dispersant being an ammonium salt.
 4. The electrostatic imaging paper of claim 1 having applied over said continuous coating of dielectric material a second coating deposited from a second aqueous dispersion of a polymeric material, in which said polymeric material is selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, containing no ionic or ionizable groups and having a low affinity for water.
 5. The electrostatic imaging paper of claim 4 wherein said second aqueous dispersion has a minimum film-forming temperature higher than 30*C.
 6. The electrostatic imaging paper of claim 4 wherein said second aqueous dispersion has a minimum film-forming temperature between 60* and 120*C.
 7. The electrostatic imaging paper of claim 4 wherein said second coating contains an inorganic pigment material in an amount up to about 20 percent by weight of dry polymeric material, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.
 8. The electrostatic imaging paper of claim 7 wherein said second coating also contains a pigment dispersant, said pigment dispersant being an ammonium salt.
 9. The electrostatic imaging paper of claim 1 wherein said polymeric material contains no ionic or ionizable groups and has a low affinity for water.
 10. The electrostatic imaging paper of claim 1 wherein said polymeric material is selected from the group consisting of acrylic and styrene-acrylic polymeric material.
 11. A method of manufacturing an electrostatic imaging paper comprising an electrically conductive base paper having on one side thereof a continuous coating of dielectric material, said method including the steps of (a) applying to one side of an electrically conductive base paper an aqueous dispersion which has a minimum film-forming temperature not greater than 30*C, wherein said polymeric material is selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, and (b) drying the coated base paper to form thereon a continuous coating of the polymeric material.
 12. The method of claim 11 wherein said aqueous dispersion of polymeric material contains an inorganic piGment in an amount up to about 20 percent by weight of dry polymeric material used, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.
 13. The method of claim 12 wherein a pigment dispersant is used to aid dispersion of the pigment material in said aqueous dispersion of polymeric material, said pigment dispersant being an ammonium salt.
 14. The method of claim 11 wherein said base paper is (a) further coated with a second aqueous dispersion of a polymeric material, said polymeric material being selected from the group consisting of polyolefins, substituted polyolefins and condensation polymers, and (b) dried to form thereon a second continuous coating of dielectric material.
 15. The method of claim 14 wherein said second aqueous dispersion has a mimimum film-forming temperature higher than 30*C.
 16. The method of claim 14 wherein said second aqueous dispersion has a minimum film-forming temperature of between 60* and 120*C.
 17. The method of claim 14 wherein said second aqueous dispersion contains an inorganic pigment material in an amount up to about 20 percent by weight of dry polymeric material, said inorganic pigment material being selected from the group consisting of titanium dioxide, barium sulfate, silica, clay and zinc oxide.
 18. The method of claim 17 wherein a pigment dispersant is used to aid dispersion of the pigment material in said second aqueous dispersion of said polymeric material, said pigment dispersant being an ammonium salt.
 19. The method of claim 11 wherein said polymeric material contains no ionic or ionizable groups and has a low affinity for water.
 20. The method of claim 11 wherein the coated base paper is dried at a temperature of substantially 100*C.
 21. The method of claim 14 wherein said polymeric material in said second aqueous dispersion contains no ionic or ionizable groups and has a low affinity for water.
 22. The method of claim 14 wherein said coated paper is dried at a temperature of substantially 120*C.
 23. The method of claim 11 wherein said polymeric material is selected from the group consisting of acrylic and styrene-acrylic polymeric material. 